Storage and File Structure

1. Storage and File Structure

2. Outline • Overview of Physical Storage Media • Magnetic Disks • RAID • Tertiary Storage • Storage Access • File Organization • Organization of Records in Files • Data-Dictionary Storage

3. Classification of Physical Storage Media • Classify storage based on • Speed with which data can be accessed • Cost per unit of data • Reliability • data loss on power failure or system crash • physical failure of the storage device • Can differentiate storage into: • volatile storage • loses contents when power is switched off • non-volatile storage: • Contents persist even when power is switched off. • Includes secondary and tertiary storage, as well as batter-backed up main-memory.

4. Storage Hierarchy

5. Storage Hierarchy • primary storage • Fastest media but volatile (cache, main memory). • secondary storage • next level in hierarchy, non-volatile, moderately fast access time • also called on-line storage • E.g. flash memory, magnetic disks • tertiary storage • lowest level in hierarchy, non-volatile, slow access time • also called off-line storage • E.g. magnetic tape, optical storage

6. Physical Storage Media • Cache • fastest and most costly form of storage; volatile; managed by the computer system hardware. • Main memory: • fast access (10s to 100s of nanoseconds) • generally too small (or too expensive) to store the entire database • capacities of up to a few Gigabytes widely used currently • Capacities have gone up and per-byte costs have decreased steadily and rapidly (roughly factor of 2 every 2 to 3 years) • Volatile • contents of main memory are usually lost if a power failure or system crash occurs.

7. Physical Storage Media • Flash memory • Data survives power failure • Data can be written at a location only once, but location can be erased and written to again • Can support only a limited number of write/erase cycles. • Erasing of memory has to be done to an entire bank of memory • Reads are roughly as fast as main memory • Writes are slow (few microseconds), erase is slower • Cost per unit of storage roughly similar to main memory • Widely used in embedded devices such as digital cameras • also known as EEPROM (Electrically Erasable Programmable Read-Only Memory)

8. Physical Storage Media • Magnetic-disk • Data is stored on spinning disk, and read/written magnetically • Primary medium for the long-term storage of data; typically stores entire database. • Data must be moved from disk to main memory for access, and written back for storage • direct-access • possible to read data on disk in any order, unlike magnetic tape • Capacities range up to roughly 200 GB currently • Much larger capacity and cost/byte than main memory/flash memory • Growing constantly and rapidly with technology improvements (factor of 2 to 3 every 2 years) • Survives power failures and system crashes • disk failure can destroy data, but is very rare

9. Physical Storage Media • Optical storage • non-volatile, data is read optically from a spinning disk using a laser • CD-ROM (640 MB) and DVD (4.7 to 17 GB) most popular forms • Write-one, read-many (WORM) optical disks used for archival storage (CD-R and DVD-R) • Multiple write versions also available (CD-RW, DVD-RW, and DVD-RAM) • Reads and writes are slower than with magnetic disk • Juke-box systems, with large numbers of removable disks, a few drives, and a mechanism for automatic loading/unloading of disks available for storing large volumes of data

10. Physical Storage Media • Tape storage • non-volatile, used primarily for backup (to recover from disk failure), and for archival data • sequential-access– much slower than disk • very high capacity (40 to 300 GB tapes available) • tape can be removed from drive  storage costs much cheaper than disk, but drives are expensive • Tape jukeboxes available for storing massive amounts of data • hundreds of terabytes to even a petabyte

11. Magnetic Hard Disk Mechanism

12. Magnetic Disks • Read-write head • Positioned very close to the platter surface (almost touching it) • Reads or writes magnetically encoded information. • Surface of platter divided into circular tracks • Over 16,000 tracks per platter on typical hard disks • Each track is divided into sectors. • A sector is the smallest unit of data that can be read or written. • Sector size typically 512 bytes • Typical sectors per track: 200 (on inner tracks) to 400 (on outer tracks) • To read/write a sector • disk arm swings to position head on right track • platter spins continually; data is read/written as sector passes under head • Head-disk assemblies • multiple disk platters on a single spindle (typically 2 to 4) • one head per platter, mounted on a common arm. • earlier generation disks were susceptible to head-crashes • Cylinder iconsists of ith track of all the platters

13. Magnetic Disks • Disk controller • interfaces between the computer system and the disk drive hardware. • accepts high-level commands to read or write a sector • initiates actions such as moving the disk arm to the right track and actually reading or writing the data • Computes and attaches checksums to each sector to verify that data is read back correctly • If data is corrupted, with very high probability stored checksum won’t match recomputed checksum • Ensures successful writing by reading back sector after writing it • Performs remapping of bad sectors

14. Disk Subsystem • Multiple disks connected to a computer system through a controller • Controllers functionality (checksum, bad sector remapping) often carried out by individual disks; reduces load on controller • Disk interface standards families • ATA (AT adaptor) range of standards • SCSI (Small Computer System Interconnect) range of standards • Several variants of each standard (different speeds and capabilities)

15. Performance Measures of Disks • Access time – the time it takes from when a read or write request is issued to when data transfer begins. Consists of: • Seek time – time it takes to reposition the arm over the correct track. • Average seek time is 1/2 the worst case seek time. • Would be 1/3 if all tracks had the same number of sectors, and we ignore the time to start and stop arm movement • 4 to 10 milliseconds on typical disks • Rotational latency – time it takes for the sector to be accessed to appear under the head. • Average latency is 1/2 of the worst case latency. • 4 to 11 milliseconds on typical disks (5400 to 15000 r.p.m.) • Data-transfer rate– the rate at which data can be retrieved from or stored to the disk. • 4 to 8 MB per second is typical • Multiple disks may share a controller, so rate that controller can handle is also important • E.g. ATA-5: 66 MB/second, SCSI-3: 40 MB/s • Fiber Channel: 256 MB/s

16. Performance Measures of Disks • Mean time to failure (MTTF) – the average time the disk is expected to run continuously without any failure. • Typically 3 to 5 years • Probability of failure of new disks is quite low, corresponding to a“theoretical MTTF” of 30,000 to 1,200,000 hours for a new disk • E.g., an MTTF of 1,200,000 hours for a new disk means that given 1000 relatively new disks, on an average one will fail every 1200 hours • MTTF decreases as disk ages

17. Optimization of Disk-Block Access • Block– a contiguous sequence of sectors from a single track • data is transferred between disk and main memory in blocks • sizes range from 512 bytes to several KB • Smaller blocks: more transfers from disk • Larger blocks: more space wasted due to partially filled blocks • Typical block sizes today range from 4 to 16 KB • Disk-arm-scheduling algorithms order pending accesses to tracks so that disk arm movement is minimized • elevator algorithm: move disk arm in one direction (from outer to inner tracks or vice versa), processing next request in that direction, till no more requests in that direction, then reverse direction and repeat

18. Optimization of Disk-Block Access • File organization – optimize block access time by organizing the blocks to correspond to how data will be accessed • E.g. store related information on the same or nearby cylinders. • Files may get fragmented over time • E.g. if data is inserted to/deleted from the file • Or free blocks on disk are scattered, and newly created file has its blocks scattered over the disk • Sequential access to a fragmented file results in increased disk arm movement • Some systems have utilities to defragment the file system, in order to speed up file access

19. Optimization of Disk-Block Access • Nonvolatile write buffers speed up disk writes by writing blocks to a non-volatile RAM buffer immediately • Non-volatile RAM: battery backed up RAM or flash memory • Even if power fails, the data is safe and will be written to disk when power returns • Controller then writes to disk whenever the disk has no other requests or request has been pending for some time • Database operations that require data to be safely stored before continuing can continue without waiting for data to be written to disk • Writes can be reordered to minimize disk arm movement • Log disk – a disk devoted to writing a sequential log of block updates • Used exactly like nonvolatile RAM • Write to log disk is very fast since no seeks are required • No need for special hardware (NV-RAM) • File systems typically reorder writes to disk to improve performance

20. RAID • RAID: Redundant Arrays of Independent Disks • disk organization techniques that manage a large numbers of disks, providing a view of a single disk of • high capacity and high speed by using multiple disks in parallel, and • high reliability by storing data redundantly, so that data can be recovered even if a disk fails • The chance that some disk out of a set of N disks will fail is much higher than the chance that a specific single disk will fail. • Originally a cost-effective alternative to large, expensive disks • I in RAID originally stood for ``inexpensive’’ • Today RAIDs are used for their higher reliability and bandwidth. • The “I” is interpreted as independent

21. Improvement of Reliability via Redundancy • Redundancy • store extra information that can be used to rebuild information lost in a disk failure • Mirroring(orshadowing) • Duplicate every disk. Logical disk consists of two physical disks. • Every write is carried out on both disks • Reads can take place from either disk • If one disk in a pair fails, data still available in the other • Mean time to data loss depends on mean time to failure, and mean time to repair • E.g. MTTF of 100,000 hours, mean time to repair of 10 hours gives mean time to data loss of 500*106 hours (or 57,000 years) for a mirrored pair of disks (ignoring dependent failure modes)

22. Improvement in Performance via Parallelism • Two main goals of parallelism in a disk system: • Load balance multiple small accesses to increase throughput • Parallelize large accesses to reduce response time. • Improve transfer rate by striping data across multiple disks. • Block-level striping • with n disks, block i of a file goes to disk (i mod n) + 1 • Requests for different blocks can run in parallel if the blocks reside on different disks • A request for a long sequence of blocks can utilize all disks in parallel • Bit-level striping – split the bits of each byte across multiple disks instead of blocks • There are multiple types of RAID: level 0 through level 6 based on various combinations of stripping and mirroring/parity

23. Storage Access • A database file is partitioned into fixed-length storage units called blocks. Blocks are units of both storage allocation and data transfer. • Database system seeks to minimize the number of block transfers between the disk and memory. • To reduce the number of disk accesses try to keep as many blocks as possible in main memory. • Buffer • portion of main memory available to store copies of disk blocks. • Buffer manager • subsystem responsible for managing buffer space in main memory.

24. Buffer Manager • Call the buffer manager when you need a block from disk • If the block is already in the buffer, return the address of the block in main memory • If the block is not in the buffer, • allocate space in the buffer for the block, replacing (throwing out) some other block, if required, to make space for the new block. • The block that is thrown out is written back to disk only if it was modified since the most recent time that it was written to/fetched from the disk. • Once space is allocated in the buffer, read the block from the disk to the buffer, and proceed as in 1 above.

25. Buffer Replacement Policies • Replace the block least recently used (LRUstrategy) • Premise: past pattern of block references is a predictor of future references • Queries have well-defined access patterns (such as sequential scans), and a database system can use the information in a user’s query to predict future references • LRU can be a bad strategy for certain access patterns involving repeated scans of data • e.g. when computing the join of 2 relations r and s by a nested loops for each tuple tr of r do for each tuple ts of s do if the tuples tr and ts match … • Mixed strategy with hints on replacement strategy providedby the query optimizer is preferable

26. Buffer-Replacement Policies • Pinned block • memory block that is not allowed to be written back to disk. • Toss-immediate strategy • frees the space occupied by a block as soon as the final tuple of that block has been processed • Most recently used (MRU) strategy • pin the block currently being processed. After the final tuple of that block has been processed, the block is unpinned, and it becomes the most recently used block. • Buffer manager can use statistical information regarding the probability that a request will reference a particular relation • E.g., the data dictionary is frequently accessed. • Heuristic: keep data-dictionary blocks in main memory buffer • Buffer managers also support forced output of blocks for the purpose of recovery

27. File Organization • The database is stored as a collection of files. • Each file is a sequence of records. • A record is a sequence of fields. • One approach: • assume record size is fixed • each file has records of one particular type only • different files are used for different relations This case is easiest to implement; will consider variable length records later.

28. Fixed-Length Records • Simple approach: • Store record i starting from byte n  (i – 1), where n is the size of each record. • Record access is simple but records may cross blocks • Modification: do not allow records to cross block boundaries • Deletion of record i: alternatives: • shift records i + 1, . . ., n down to i, . . . , n – 1 • move record n to i • do not move records, but link all free records on a free list • Store the address of the first deleted record in the file header. • Use this first record to store the address of the 2nd deleted record, etc

29. Variable-Length Records • Variable-length records arise due to • Storage of multiple record types in a file • Records that allow variable lengths for one or more fields • Records that allow repeating fields (used in some older & newer data models). • Byte string representation • Attach an end-of-record () control character to the end of each record • Difficulty with deletion and growth • Fixed-length representation using reserved space • use fixed-length records of a known maximum length • unused space in shorter records filled with a null or end-of-record symbol. • Fixed-length representation using pointers • Use a linked list of fixed-length records

30. Variable-Length Records • Fixed-length representation using pointers

31. Variable-Length Records • Disadvantage to pointer method • space is wasted in all records except the first in a a chain. • Solution is to allow two kinds of block in file: • Anchor block – contains the first records of chain • Overflow block – contains records other than those that are the first records of chairs.

32. Slotted Page Representation • Each block that stores records has a header as above • Records can be moved around within a page to keep them contiguous with no empty space between them • entry in the header must be updated. • Pointers should point to the entry for the record in header.

33. Organization of Records in Files • Heap • a record can be placed anywhere in the file where there is space • Sequential • store records in sequential order, based on the value of the search-key of each record • Hashing • a hash on some attributes of each record specifies the block of the file to place the record should be placed • Records of each relation may be stored in a separate file. • In a clustering file organization records of several different but co-accessed relations are stored in the same file to reduce I/O

34. Sequential File Organization • Suitable for applications that require sequential processing of the entire file • The records in the file are ordered by a search-key • Link them in place in a sorted linked-list • Insertion –locate the position where the record is to be inserted • if there is free space insert it there • if no free space, insert the record in an overflow block • Deletion – use linked list of free records • Periodically reorganize the file

35. Clustering File Organization • Example clustering file organization of customer and depositor: • good for queries involving depositor customer, and for queries involving one single customer and his accounts • bad for queries involving only customer • results in variable size records

36. Data Dictionary Storage • Data dictionary stores metadata • Information about relations • names of relations • names and types of attributes of each relation • names and definitions of views • integrity constraints • User and accounting information, including passwords • Statistical and descriptive data • number of tuples in each relation • Physical file organization information • How relation is stored (sequential/hash/…) • Physical location of relation • operating system file name or • disk addresses of blocks containing records of the relation • Information about indices

37. Data Dictionary Storage • Catalog structure can use either • specialized data structures designed for efficient access • a set of relations, with existing system features used to ensure efficient access • A possible catalog schema Relation-metadata = (relation-name, number-of-attributes, storage-organization, location)Attribute-metadata = (attribute-name, relation-name, domain-type, position, length) User-metadata = (user-name, encrypted-password, group) Index-metadata = (index-name, relation-name, index-type, index-attributes) View-metadata = (view-name, definition)